In the operation of a conventional gas turbine, intake air from the atmosphere is compressed and heated by a compressor and is caused to flow to a combustor, where fuel is mixed with the compressed air and the mixture is ignited and burned. The heat energy thus released then flows in the combustion gasses to the turbine where it is converted into rotary mechanical energy for driving equipment, such as for generating electrical power or for running an industrial process. The combustion gasses are then exhausted from the turbine back into the atmosphere. These gases include pollutants such as oxides of nitrogen, carbon monoxide and unburned hydrocarbons. Various schemes have been used to minimize the generation of such pollutants during the combustion process. The use of a combustion catalyst in the combustion zone is known to reduce the generation of these pollutants since catalyst-aided combustion promotes complete combustion of lean premixed fuels and can occur at temperatures well below the temperatures necessary for the production of NOx species. Typical catalysts for a hydrocarbon fuel-oxygen reaction include platinum, palladium, rhodium, iridium, terbium-cerium-thorium, ruthenium, osmium and oxides of chromium, iron, cobalt, lanthanum, nickel, magnesium and copper.
FIG. 1 illustrates a prior art gas turbine combustor 10 wherein at least a portion of the combustion takes place in a catalytic reactor 12. Such a combustor 10 is known to form a part of a combustion turbine apparatus such as may be used to power an electrical generator or a manufacturing process. Compressed air 14 from a compressor (not shown) is mixed with a combustible fuel 16 by a fuel-air mixing device such as fuel injectors 18 at a location upstream of the catalytic reactor 12. Catalytic materials present on surfaces of the catalytic reactor 12 react the fuel-air mixture at temperatures lower than normal ignition temperatures. Known catalyst materials are not active at the compressor discharge supply temperature for certain fuels and engine designs, such as natural gas lean combustion. Accordingly, a preheat burner 20 is provided to preheat the combustion air 14 by combusting a supply of preheat fuel 22 upstream of the main fuel injectors 18. Existing catalytic combustor designs react approximately 10-15% of the fuel on the catalyst surface, with the remaining combustion occurring downstream in the burnout region 24. Increasing the percentage of the combustion on the catalyst surface will decrease the amount of combustion occurring in the flame, thus decreasing the overall emission of oxides of nitrogen. However, increasing the amount of combustion on the catalyst surface will also increase the temperature of both the catalyst and the catalyst substrate. One of the limitations to increasing the amount of combustion in the catalytic reactor 12 is the operating temperature limit of the underlying metal substrate material.
The operating environment of a gas turbine is very hostile to catalytic reactor materials, and is becoming even more hostile as the demand for increased efficiency continues to drive firing temperatures upward. Ceramic substrates used for catalytic reactor beds are prone to failure due to thermal and mechanical shock damage. Furthermore, ceramic substrates are difficult to fabricate into complex shapes that may be desired for catalyst elements. Metal substrates have been used with some success with current generation precious metal catalysts at temperatures up to about 800° C. Such catalytic reactors are produced by applying a ceramic wash-coat and catalyst directly to the surface of a high temperature metal alloy. In one embodiment, the catalytic reactor 12 of FIG. 1 is formed as a plurality of metal tubes. The outside surfaces of the tubes are coated with a ceramic wash-coat and a platinum catalyst. The fuel-air mixture is combusted at the catalyst surface, thereby heating the metal substrate. The substrate is cooled by passing an uncombusted fuel-air mixture through the inside of the tube. Other geometries of back-cooled metal substrate catalyst modules may be envisioned, such as the catalytic combustor described in U.S. Pat. No. 4,870,824 dated Oct. 3, 1989.
U.S. Pat. No. 5,047,381 dated Sep. 10, 1991, describes a laminated substrate for a catalytic combustor reactor bed including a metal alloy substrate coated with a noble metal, such as platinum, upon which a ceramic wash-coat such as alumina is applied. A catalyst is applied with the wash-coat or individually over the wash-coat. The noble metal coating prevents oxygen from contacting the metal substrate, thereby minimizing its degradation by oxidation reactions. The underlying noble metal also acts as a catalyst in the event that a portion of the ceramic wash-coat erodes or is otherwise removed from the substrate. While the reduced rate of oxidation will extend the life of the reactor bed in a combustor at any given temperature, such a design does not offer any significant thermal protection for the substrate. Work is underway to develop catalysts operable at higher combustion temperatures. As the allowable working temperature of the catalyst increases, the task of cooling the metal substrate supporting the catalyst will become increasingly difficult.